Fixed output add button could become invisible (#6660)

* [Hack] Fixed output add button could become invisible

This can happen when the scrollview is too small and the horizontal scrollbar is not at the right most position

* Fixed an issue when there's no output category in the blackboard

* Fixed formatting issue

* Updated changelog

Author: julienamsellem

com.unity.visualeffectgraph/CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Incorrect behavior of Tangent Space in ShaderGraph [Case 1363279](https://issuetracker.unity3d.com/product/unity/issues/guid/1363279/)
 - ShaderGraph made with new VFX SG integration where not listed when searching for a shader graph output [Case 1379523](https://issuetracker.unity3d.com/product/unity/issues/guid/1379523/)
 - Enable/disable state of VFX blocks and operators are preserved after copy/paste
+- Blackboard "Add" button for output could be hidden when the panel is too small (https://issuetracker.unity3d.com/product/unity/issues/guid/1389927/)
 
 
 ## [13.1.2] - 2021-11-05

com.unity.visualeffectgraph/Editor/GraphView/Blackboard/VFXBlackboard.cs

@@ -1,4 +1,3 @@
-using System;
 using System.Linq;
 using UnityEditor.Experimental.GraphView;
 using UnityEngine;
@@ -133,6 +132,14 @@ public VFXBlackboard(VFXView view)
             m_AddButton.SetEnabled(false);
 
             this.AddManipulator(new ContextualMenuManipulator(BuildContextualMenu));
+
+            // Workaround: output category is in a scrollview which can lead to get the Add button invisible (moved out of the visible viewport of the scrollviewer)
+            var scrollView = this.Q<ScrollView>();
+            if (scrollView != null)
+            {
+                scrollView.RegisterCallback<GeometryChangedEvent, ScrollView>(OnGeometryChanged, scrollView);
+                scrollView.horizontalScroller.valueChanged += x => OnOutputCategoryScrollChanged(scrollView);
+            }
         }
 
         public void LockUI()
@@ -167,6 +174,23 @@ DropdownMenuAction.Status GetContextualMenuStatus()
             return DropdownMenuAction.Status.Disabled;
         }
 
+        void OnOutputCategoryScrollChanged(ScrollView scrollView)
+        {
+            OnGeometryChanged(null, scrollView);
+        }
+
+        void OnGeometryChanged(GeometryChangedEvent evt, ScrollView scrollView)
+        {
+            if (scrollView != null)
+            {
+                var addOutputButton = scrollView.Q<Button>("addOutputButton");
+                if (addOutputButton != null)
+                {
+                    addOutputButton.style.left = -scrollView.horizontalScroller.highValue + scrollView.horizontalScroller.value;
+                }
+            }
+        }
+
         void BuildContextualMenu(ContextualMenuPopulateEvent evt)
         {
             evt.menu.AppendAction("Select All", (a) => SelectAll(), (a) => GetContextualMenuStatus());